In a C-axis control of the angular position of the spindle motor of a machine tool for a contour machining, the spindle must be smoothly rotated at a very low rotating speed and the spindle motor must deliver a high output, and during a usual machining, the spindle motor must be operated at a high rotational speed and deliver a high output.
The conventional machine tool is provided with two motors, namely, one for low-speed machining and the other for high-speed machining, to meet those two requirements, but such a drive system of the conventional machine tool is costly and requires a large floor space. Another system employs belts and gears to maintain a high output for machining over a range of from a low-speed to a high-speed, which also requires an increased floor space.